In the majority of non-impact printing processes, toner transfer to paper substrates is accomplished by electrostatic transfer using an electrically biased roller or a corona source. Most processes operate at low speeds where the dynamic forces at the toner transfer point are such that transfer is accomplished without an appreciable scatter of toner from the image area. Higher speed systems, however, using an electrically biased roller or a pressure roller suffer from a problem with scattering of the toner at the nipping point. Such scatter is often referred to as back scatter, trailing edge dusting, or character puffing. It is caused by the physical blowing of toner from the image area due to the reversal of the trapped entrained air streams which follow the image cylinder and paper into the nipping point. The high velocity air stream going counter to the direction of the toner breaks the electrostatic adhesion force holding the toner to the image member.
The effect of blowing toner typically does not start until a particular linear speed of the paper web exceeds a certain amount. For example, with a Midax 300 electrostatic printing system, available from Moore Business Forms, Lake Forest, Illinois, serious back scatter does not take place until the speeds are greater than about 200 fpm; for a Nipson-Bull Varypress (which may include a vacuum knife preceding the nip point to lower the atmospheric pressure in the area to reduce back scatter), back scatter starts to significantly occur when speeds greater than 150 fpm are reached.
According to the present invention, a simple method and apparatus are provided which substantially eliminate the problem of back scatter even at speeds significantly in excess of 200 fpm. The desirable results according to the invention are essentially accomplished by applying an electrical potential in the vicinity of the transfer from the image member to the web of paper that has the same polarity as the toner particles on the image member, so that the combination of the adhesion force and the field force applied according to the invention overcome the aerodynamic forces, and trailing edge dusting is almost completely eliminated.
According to one aspect of the present invention a method of transferring charged toner from an image member to a web of imagable material (typically paper) is provided using an impression cylinder with a nip between the image member and impression cylinder comprising the steps of: (a) Applying toner having a first polarity to the image member. (b) Moving the web of imagable material at a linear speed in excess of about 150 fpm into contact with the image member to effect toner transfer. (c) Effecting pressure transfer of the toned image from the image member to the paper using the nip between the image member and impression cylinder. And (d) substantially preventing toner back scatter by applying an electrical bias of the first polarity in the vicinity of the area of contact between the image member and the web so as to impose an electric force field on the toner particles of sufficient intensity to overcome aerodynamic drag forces which would separate the particles from the image member before application to the web.
Step (b) is typically practiced in excess of 200 feet per minute, and step (c) is practiced by applying a fixed or varying potential of about 100-600 volts in the vicinity of the area of contact between the image member and the web, or by applying a fixed potential of about 750 volts in order to accommodate all types of toners, possible web speeds, and geometries. Steps (a) and (d) are both typically practiced to apply a positive charge, although a negative charge may also be applied in both situations.
According to another aspect of the present invention a non impact, electrostatic printing device is provided. The device comprises the following components: An image member to which toner having a first polarity is applied. A conductive impression cylinder having an outer periphery. Means for rotating the impression cylinder about an axis. The image member and impression cylinder being positioned with respect to each other to provide a nip, and so that a web of imagable material passes between the image member and the outer periphery of the impression cylinder so that toner from the image member is transferred to the web of material by pressure fixing the toner at the nip. And means for applying an electrical potential of the first polarity to the impression cylinder of sufficient intensity so as to substantially prevent back scatter of toner as a result of aerodynamic forces acting between the image member and impression cylinder.
The image member may comprise an image belt, an image cylinder, or a wide variety of other structures which are used to transfer toner to a web. Where an image cylinder is utilized, means are provided for rotating an image cylinder about an axis slightly skewed with respect to the axis of the impression cylinder, the cylinders being rotated in different directions and having the nip therebetween. The image and impression cylinders are typically rotated at such a speed that the tangential speeds thereof (and thus the speed of the web) are greater than about 200 fpm. The means for applying an electrical potential comprises any conventional power supply, and a universal potential of about 750 volts will take into account all different types of toners and speeds.
According to another aspect of the present invention, a method of transferring charged toner from an image member to a web of imagable material using an impression cylinder is provided. The method comprises the following steps: (a) Applying toner having a first polarity to the image member. (b) Moving the web of imagable material by contact with the impression cylinder into contact with the image member so that toner transfers from the image member to the web. And (c) substantially preventing toner back scatter by applying an electrical bias of said first polarity to the impression cylinder so as to impose an electric force field on the toner particles of sufficient intensity to overcome aerodynamic drag forces which would separate the particles from image areas between the impression cylinder and image member.
Typically the image member and the impression cylinder are moved at a tangential or linear speed greater than about 200 fpm, and step (c) is practiced by applying a fixed or varying potential of about 100-600 volts, more preferably a fixed potential of about 300-400 volts, although a potential of about 750 volts takes care of all varieties of toners and speeds. The web of imagable material is typically paper.
It is a primary object of the present invention to hold charged toner in a desired position with respect to an image member until the web to which transfer is to take place is in direct contact with the toner so as to substantially eliminate back scatter. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.